Basically fragment obtained by Reverse transcriptase PCR on expressed mRNA are sequenced with your method of choice. Since they can be quite long (sanger method gives 500 to 800 nt), their sequence is unique and can then be detected in the genome. This allow the placement of expressed genes in the different chromosomes.

I you want more details, you can start from the eil wikipedia and look at the reference therin or come back with more detailed questions

Good luck

Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)